Three Challenges of Formulating with Mild Surfactants

Usage of mild surfactants in skin-cleansing products, especially amino-acid-based surfactants, has steadily increased in the past decade. It has shown a dramatic increase since 2014. The key driving force comes from high demands on healthy cleansing products with attractive sensory qualities. Mild surfactants provide low skin irritation and luxurious creamy foam at the beginning of washing. They are easy to rinse off but don’t squeak during washing and have a soft, moisturizing and smooth after-feel. The well-known challenge to formulate mild surfactants into skin-cleansing products is to balance thickening and speed of foaming. Some of the available thickening technologies could reach target viscosity, but often the required usage level is so high that they can potentially suppress the foam or make the foaming process difficult. In other cases, some technologies can provide thickening but can barely keep a stable viscosity at different temperatures—it becomes watery at a high temperature and jelly at a low temperature.

Challenge #1: Improving Thickening Efficiency

To achieve viscosity, the main existing thickening technologies on the market are conventional thickeners or associative thickeners. Conventional thickeners are most often water-soluble polymers of high molecular weight that rarely contain hydrophobic groups, like xanthan gum or cellulose esters. The mechanism of thickening comes from chain entanglement, hydrodynamic volume and flocculation of dispersed components. These high-molecular weight polymers can occupy big amounts of water, limiting the motion of water and therefore leading to the viscosity increase. Temperature and polymer dosage will significantly impact the bonded water content; therefore potential risk can be observed due to not forming a stable homogenous system, like in Figure 1, phase separation, or the systems are sensitive to temperature fluctuations.

Another category is associative thickeners having hydrophobic and hydrophilic groups. The representative includes polyethylene glycol esters such as PEG-150 Distearate, hydrophobic modified cellulose, hydrophobic modified polysaccharide associative thickeners, hydrophobic modified acrylates copolymer or hydrophobically modified alkaline soluble emulsion (HASE). The polymer is hydrated and forms hydrophobic interaction with surfactant micelle to establish the network and achieve thickening. Normally it requires a high level of nonionic associative thickener, such as PEG-150 Distearate, to thicken a mild surfactant system. Compared with a conventional sodium laureth sulfate (SLES) system, a mild surfactant has a big hydrophilic head, so it is not easy to form surfactant micelles. The hydrophobic interaction between polymer and surfactant micelle becomes the key driving force to build up the viscosity and stabilize the system.

The viscosity consistency at different temperatures is another key required performance that formulators are pursuing. As shown in Figure 2, you can see a summary of the comparison of commercialized rheology modifiers. The temperature impacts the water bonding ability and the associative interaction; therefore polymers such as PEG-150 Distearate or HASE show a big variation between high and low temperatures, which leads to a gelling effect at low temperatures and a significant loss of viscosity at high temperatures. This is not acceptable to consumers, especially when they use the product in different seasons. Charge repulsion, hydrophobic interaction and hydrogen bonding are the three main forces formed by a polyacrylate crosspolymer; therefore it is not sensitive to temperature variations.

Sensory qualities of a body wash formulation are a key performance that consumers can perceive directly. Both crosspolymer-1 and -2 are able to enhance foam creaminess with no negative influence on flash foam (foaming speed), as shown in Figure 3. However, normally thickeners will suppress foam volume as well as foaming speed when adding a certain amount to achieve required viscosity.

Other Challenges: Achieving Effective Suspension Power

Albeit thickening and viscosity consistency are two main challenges for mild surfactant systems, when you want to expand a product line to develop eye-catching products with suspended functional or colorful beads, high suspension power for the formula should never be neglected. Compared with different rheology modifiers or thickeners, crosslinked polymers provide good suspension power. As shown in Figure 4, crosspolymer-1 at 0.8 percent in glycinate surfactant blends can suspend different beads.

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